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These will sometimes reset if you turn them off for 10 minutes then turn them back on... or if you unplug them for a few minutes. If that fails it it is going to take Rinnai service to arrive and do repairs.
If that turns out to be the case, tell me which direction you are from the center of the nearest city and I will see if I can find a well rated Rinnai tech in your area.
We can go from there.